The present invention relates to a ceramic comprising (or consisting essentially of) a solid solution containing Bi, K, Ti and Fe (and optionally Pb) which exhibits piezoelectric behaviour.
Piezoelectric materials generate an electric field in response to applied mechanical strain. The effect is attributable to a change of polarization density within the material. The piezoelectric effect is reversible in the sense that stress or strain is induced when an electric field is applied to the material. These properties are deployed in piezoelectric sensors and actuators which are used widely in a number of specific applications and instruments. Examples of the use of piezoelectric materials include medical ultrasound and sonar, acoustics, vibration control, spark igniters and (more recently) diesel fuel injection.
The family of ceramics with a perovskite or tungsten-bronze structure exhibits piezoelectric behaviour. There are a number of examples of ceramics with a perovskite structure which are lead-containing. For example, lead zirconate titanate (Pb[ZrxTi1-x]O3 0<x<1) which is more commonly known as PZT exhibits a marked piezoelectric effect and is the most common piezoelectric ceramic in use today. However it has a maximum operating temperature of only about 200° C. WO-A-2006/032872 discloses the lead-containing perovskite compound (BiFeO3)x—(PbTiO3)1-x which exhibits piezoelectric behaviour.
There is growing concern over the toxicity of lead-containing devices and this concern is reflected in environmental regulation and policy. As a result there is increasing interest in lead-free piezoelectric materials. Known lead-free piezoelectric materials are NaNbO3, BiFeO3, (Bi1/2Na1/2)TiO3, (Bi1/2K1/2)TiO3, BaTiO3, KNbO3 and solid solutions such as (Bi1/2Na1/2)TiO3—(Bi1/2K1/2)TiO3—BaTiO3, (Bi1/2Na1/2)TiO3—(Bi1/2K1/2)TiO3—BiFeO3 (see Zhou et al, Mat. Chem. & Phys., 114, 2009, 832-836), (Bi1/2Na1/2)TiO3—(Bi1/2K1/2)TiO3, (Bi1/2K1/2)TiO3—BaTiO3, (Bi1/2Na1/2)TiO3—BaTiO3, (Bi1/2Na1/2)TiO3—BaTiO3—BiFeO3 (see Nagata et al, Ferroelectrics, 229, Issue 1 May 1999, 273-278) and (K, Na)NbO3.
JP-2008-069051 discloses a piezoelectric ceramic containing x(BiaK1-a)TiO3—(1−x)BiFeO3 and substantial proportions of non-perovskite ternary oxides such as Bi2Fe4O9 and Bi3Ti4O12. The highest Curie point was reported to be 480° C. for the ceramic in which x is 0.3.
The present invention is based on the recognition that solid solutions of formula x(BiaK1-a)TiO3-yBiFeO3-zPbTiO3 (such as x(BiaK1-a)TiO3-(1−x)BiFeO3 (hereinafter xKBT-1-xBF)) which are substantially free of non-perovskite phases typically exhibit a high Curie point and/or excellent piezoelectric activity.
Thus viewed from a first aspect the present invention provides a ceramic comprising (eg consisting essentially of or consisting of) a solid solution of formula:x(BiaK1-a)TiO3-yBiFeO3-zPbTiO3                 wherein 0.4≤a≤0.6;        0<x<1;        0<y<1;        0≤z≤0.5; and        x+y+z=1,wherein the ceramic is substantially free of non-perovskite phases.        
Typically the ceramic of the invention advantageously exhibits a Curie point in excess of 350° C. but often 700° C. or more.
Preferably the ceramic consists essentially of the solid solution. For example, the solid solution may be present in the ceramic in an amount of 50 wt % or more (eg in the range 50 to 99 wt %), preferably 75 wt % or more, particularly preferably 90 wt % or more, more preferably 95 wt % or more.
Preferably the ceramic further comprises one or more perovskite phases. Particularly preferably the (or each) perovskite phase is selected from the group consisting of (BiaK1-a)TiO3, BiTiO3, KTiO3, BiFeO3 and PbTiO3. The (or each) perovskite phase may be present in an amount of 75 wt % or less, preferably 50 wt % or less, particularly preferably 25 wt % or less, more preferably 5 wt % or less. The (or each) perovskite phase may be present in a trace amount.
The non-perovskite phases may be mixed metal phases of two or more (eg three) of Bi, K, Ti, Fe or Pb. Examples include Bi2O3, K2O, Bi2Fe4O9 and Bi3Ti4O12.
The amount of non-perovskite phases present in the ceramic may be such that the phases are non-discernible in an X-ray diffraction pattern. The amount of non-perovskite phases present in the ceramic may be a trace amount.
Preferably the total amount of non-perovskite phases present in the ceramic is less than 10 wt %, particularly preferably less than 8 wt %, more preferably less than 5 wt %, yet more preferably less than 2 wt %, still yet more preferably less than 1 wt %, most preferably less than 0.1 wt %.
The solid solution may be a partial solid solution. Preferably the solid solution is a complete solid solution.
The solid solution may be substantially monophasic.
The solid solution may be biphasic. Preferably the solid solution has two of the group consisting of a rhombohedral phase, a monoclinic phase, an orthorhombic phase and a tetragonal phase. The solid solution may have a rhombohedral phase and a monoclinic phase. The solid solution may have a rhombohedral phase and orthorhombic phase. Preferably the solid solution has a tetragonal phase and a rhombohedral phase.
Preferably 0≤z≤0.3.
In a preferred embodiment, z is 0. Preferably in this embodiment the ceramic comprises (eg consists essentially of or consists of) a solid solution of formula:x(BiaK1-a)TiO3-(1−x)BiFeO3 wherein a is in the range 0.4 to 0.6 and x is in the range 0.01 to 0.99, wherein the ceramic is substantially free of non-perovskite phases.
The solid solution may be a solid solution of (BiaK1-a)TiO3 in BiFeO3. The solid solution may be a solid solution of BiFeO3 in (BiaK1-a)TiO3.
Preferably x is in the range 0.1 to 0.9
Particularly preferably x is in the range 0.7 to 0.9. Particularly preferred in this range is a biphasic solid solution of a tetragonal and rhombohedral phase.
Particularly preferably x is in the range 0.1 to 0.4. The ceramics in this range exhibit a surprisingly high Curie point and are potentially useful in high temperature environments.
Particularly preferably x is in the range 0.5 to 0.6.
Preferably a is in the range 0.45 to 0.55. Particularly preferably a is in the range 0.48 to 0.52. More preferably a is 0.50.
In the solid solution, one or more of Bi, K, Fe and Ti may be substituted by a metal dopant. The metal dopant for each substitution may be the same or different. The presence of a metal dopant may significantly and unpredictably impact on the properties of the solid solution. For example, there may be an improvement in the Curie point and/or the piezoelectric activity.
The (or each) metal dopant may be present in an amount up to 50 at %, preferably up to 20 at %, particularly preferably up to 10 at %, more particularly preferably up to 5 at %, yet more preferably up to 3 at %, most preferably up to 1 at %.
The metal dopant may be an A-site metal dopant. For example, the A-site metal dopant may substitute Bi and/or K. Preferably the A-site metal dopant is selected from the group consisting of Li, Na, Ca, Sr, Ba and a rare earth metal.
The metal dopant may be a B-site metal dopant. For example, the B-site metal dopant may substitute Fe and/or Ti.
A preferred A-site metal dopant is Li or Na. The substitution of Li or Na on the A-site may modify (eg increase) the Curie point and/or favourably shift the phase composition of any biphasic solid solution (eg rhombohedral-tetragonal solid solution).
A preferred A-site metal dopant is Ca, Sr or Ba. The substitution of Ca, Sr or Ba on the A-site may reduce dielectric loss, modify (eg increase) the Curie point and/or favourably shift the phase composition of any biphasic solid solution (eg rhombohedral-tetragonal solid solution).
A preferred A-site metal dopant is a rare earth metal. A particularly preferred A-site metal dopant is La or Nd. Typically La or Nd substitute K. Substitution by La or Nd may increase the piezoelectric activity at the expense of the Curie point. By way of example (for a given BiFeO3 concentration), substitution by La and Nd would typically reduce the Curie point by about 100-200° C. and increase the piezoelectric activity by 50%.
In a particularly preferred embodiment, the A-site metal dopant is La (eg La3+) which substitutes K (ie K+). This substitution may improve significantly the resistivity.
A preferred B-site metal dopant has a higher valency than the valency of the metal which it substitutes. Conductivity in perovskites is usually attributable to electron holes or oxygen vacancies. Substituting a higher valence metal dopant onto a B-site may enhance appreciably the resistivity (ie suppress the conductivity).
In a particularly preferred embodiment, the B-site metal dopant has a valency in the range IV to VII. More particularly preferred is a B-site metal dopant selected from the group consisting of Ti, Zr, W, Nb, V, Ta, Mo and Mn. Yet more particularly preferred is a B-site metal dopant selected from the group consisting of Nb, Ta, Mo, W, Zr and V.
A preferred B-site metal dopant is selected from the group consisting of Ti, Fe, Co and Ni. Particularly preferred is Ti (eg Ti4+) which substitutes Fe (ie Fe3+).
In a preferred embodiment, the B-site metal dopant has a mixed valency. Substituting a mixed valency metal dopant onto a B-site may improve the resistivity significantly.
In a particularly preferred embodiment, the B-site metal dopant is Mn. An advantage of Mn is that it behaves as a buffer in the sense that it can adopt a range of oxidation states which can improve resistivity in a range of ceramics.
In a particularly preferred embodiment, the B-site metal dopant is Co. Typically Co substitutes Fe (ie Fe3+).
The ceramic may take the form of a textured ceramic, a single crystal, a thin film or a composite (eg a ceramic/glass or ceramic/polymer composite).
Preferably the Curie point of the ceramic is 350° C. or more, particularly preferably 400° C. or more, more preferably 700° C. or more.
Preferably the ceramic has an X-ray diffraction pattern substantially as illustrated in FIG. 2 or 7.
The ceramic may be obtainable by sintering a sinterable form of a mixed metal oxide containing Bi, K, Fe and Ti (and optionally Pb) to produce the ceramic.
Preferably the ceramic further comprises a pre-sintering additive.
The pre-sintering additive may be present in an amount of 75 wt % or less, preferably 50 wt % or less, particularly preferably 25 wt % or less, more preferably 5 wt % or less. The pre-sintering additive may be present in a trace amount.
The pre-sintering additive may be a perovskite. The pre-sintering additive may be a layered perovskite such as Bi4Ti3O12. The pre-sintering additive may be a lead-containing perovskite. The lead-containing perovskite may be PbTiO3 or PbZrO3.
The pre-sintering additive may be added post-reaction (eg post-calcination) to form the mixed metal oxide containing Bi, K, Fe and Ti (and optionally Pb). The pre-sintering additive may serve as a sintering aid.
In a preferred embodiment, the ceramic is obtainable by a process comprising:                (A) preparing an intimate mixture of a substantially stoichiometric amount of a compound of each of Bi, K, Ti and Fe (and optionally Pb);        (B) converting the intimate mixture into an intimate powder;        (C) inducing a reaction in the intimate powder to produce a mixed metal oxide;        (D) manipulating the mixed metal oxide into a sinterable form; and        (E) sintering the sinterable form of the mixed metal oxide to produce the ceramic.        
Viewed from a yet further aspect the present invention provides a process for preparing a ceramic as hereinbefore defined comprising:                (A) preparing an intimate mixture of a substantially stoichiometric amount of a compound of each of Bi, K, Ti and Fe (and optionally Pb);        (B) converting the intimate mixture into an intimate powder;        (C) inducing a reaction in the intimate powder to produce a mixed metal oxide;        (D) manipulating the mixed metal oxide into a sinterable form; and        (E) sintering the sinterable form of the mixed metal oxide to produce the ceramic.        
In step (A), it may be advantageous for one or more of the compounds of Fe, Ti, K and Bi (and optionally Pb) to depart from a stoichiometric amount. For example, one or more of Fe, Ti, K and Bi (and optionally Pb) is present in excess of the stoichiometric amount. For example, the atomic % may depart from stoichiometry by ±20% or less, preferably ±10% or less, particularly preferably ±5% or less. By departing from stoichiometry, the ceramic may be equipped advantageously with useful oxide phases (eg perovskite phases).
Preferably in step (A) the substantially stoichiometric amount of the compound of each of Bi, K, Ti and Fe (and optionally Pb) is expressed by the compositional formula:x(BibKc)TiO3-y(BiFe1-dBdO3)-zPbTiO3 wherein:                B is a B-site metal dopant as defined hereinbefore;        b is in the range 0.4 to 0.6;        c is in the range 0.4 to 0.6;        d is in the range 0 to 0.5; and        x, y and z are as hereinbefore defined.        
In a particularly preferred embodiment, B is Ti.
In a particularly preferred embodiment, B is Co.
In a particularly preferred embodiment, d is in the range 0 to 0.2. More preferably d is zero.
In a particularly preferred embodiment, z is zero.
In a particularly preferred embodiment, b is a as hereinbefore defined and c is (1−a).
Step (A) may include a metal dopant oxide which delivers a metal dopant as hereinbefore defined.
The compound of each of Bi, K, Ti and Fe (and optionally Pb) may be independently selected from the group consisting of an oxide, nitrate, hydroxide, hydrogen carbonate, isopropoxide, polymer and carbonate, preferably an oxide and carbonate. Examples are Bi2O3 and K2CO3.
The intimate mixture may be a slurry (eg a milled slurry), a solution (eg an aqueous solution), a suspension, a dispersion, a sol-gel or a molten flux.
Step (C) may include heating (eg calcining). Preferably step (C) includes stepwise or interval heating. Step (C) may include stepwise or interval cooling.
Where the intimate mixture is a solution, the compound may be a salt (eg a nitrate).
Where the intimate mixture is a sol-gel, the compound may be an isopropoxide.
Where the intimate mixture is a molten flux, the compound may be an oxide dissolved in a salt flux. The mixed metal oxide from step (C) may be precipitated out on cooling.
Preferably the intimate powder is a milled powder. Step (A) may be:                (A1) preparing a slurry of a substantially stoichiometric amount of a compound of each of Bi, K, Ti and K (and optionally Pb);        (A2) milling the slurry;and step (B) may be        (B1) drying the slurry to produce the milled powder.        
Step (E) may be stepwise or interval sintering. Preferably step (E) includes stepwise or interval heating and stepwise or interval cooling.
Step (E) may be carried out in the presence of a sintering aid. The presence of a sintering aid promotes densification. The sintering aid may be CuO2.
Step (D) may include milling the mixed metal oxide. Step (D) may include pelletising the mixed metal oxide.
Viewed from a still yet further aspect the present invention provides the use of a ceramic as hereinbefore defined in a piezoelectric device.
Preferably in the use according to the invention the piezoelectric device is operable at a temperature in excess of 400° C.
The piezoelectric device may be a piezoelectric actuator, sensor or transformer. For example the piezoelectric device may be an industrial steam sensor.
Preferably in the use according to the invention the piezoelectric device is deployed in an aero-engine.